A humanized monoclonal antibody against the Respiratory Syncytial Virus (RSV) fusion protein (MEDI-493) will be assessed for safety and efficacy in a double-blind, randomized prophylaxis trial. Preterm infants with and without Broncho-pulmonary Dysplasia (BPD) will receive 3-5 monthly IM injections of MEDI-493 or placebo during RSV season. Endpoints include (1) the incidence of RSV infection and (2) the number of RSV hospitalizations.